(Reuters) - NATO's top military commander said on Sunday Russia had built up a "very sizeable" force on its border with Ukraine and Moscow may have a region in another ex-Soviet republic, Moldova, in its sights after annexing Crimea.

Russia was acting more like an adversary than a partner, NATO's Supreme Allied Commander Europe, U.S. Air Force General Philip Breedlove said, and the 28-nation alliance should rethink the positioning and readiness of its forces in eastern Europe.

Russian troops, using armored vehicles, automatic weapons and stun grenades, seized some of the last military facilities under Ukrainian control on Saturday in Crimea, the Black Sea peninsula that Russian President Vladimir Putin formally annexed the day before.

Breedlove was one of several Western officials and politicians to warn on Sunday that Russia may not stop there in a crisis that has taken East-West relations lurching back towards the Cold War since pro-Western protests in Ukraine ousted Moscow-allied President Viktor Yanukovich last month.

"The (Russian) force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," the NATO commander told an event held by the German Marshall Fund think-tank.

U.S. President Barack Obama's deputy national security adviser Tony Blinken said the build-up might just be aimed at intimidating Ukraine's new pro-Western leaders but that Russia could invade the country's mainly Russian-speaking east. "It's possible that they are preparing to move in," he told CNN.

Russia said it was complying with international agreements and had no plans to invade. It has called the soldiers who took over Ukrainian bases in Crimea "self defense forces".

The United States and the European Union have targeted some of Putin's closest political and business allies with personal sanctions and have threatened broader economic sanctions if Putin's forces encroach on other eastern or southern parts of Ukraine with big Russian-speaking populations.

Ukrainian marine standards were still flying on Sunday alongside the Russian flag at the Crimean base of Ukraine's top military unit in Fedosiya, but the Ukrainian troops were getting ready to leave after the Russian military takeover.

"Our only issue is that we want to leave this place with honor, weapons and vehicles," one Ukrainian soldier said.

Blinken said Washington was considering all requests for military assistance from the government in Kiev, but that it would be unlikely to prevent an invasion of Ukraine, which is not part of NATO. Breedlove said the military alliance needed to think about its eastern members, particularly the former Soviet Baltic states of Lithuania, Latvia and Estonia.

"We need to think about our allies, the positioning of our forces in the alliance and the readiness of those forces ... such that we can be there to defend against it if required, especially in the Baltics and other places," Breedlove said.

"VERY WORRISOME"

Breedlove said NATO was very concerned about the threat to Transdniestria, which declared independence from Moldova in 1990 but has not been recognized by any United Nations member state. About 30 percent of its half million population is ethnic Russian, which is the mother tongue of an overall majority.

"There is absolutely sufficient (Russian) force postured on the eastern border of Ukraine to run to Transdniestria if the decision was made to do that, and that is very worrisome," Breedlove said.

The speaker of Transdniestria's parliament has urged Russia to incorporate the region, which lies to the west of Ukraine. The new leaders in Kiev have said Moscow could seek to link up pro-Russian regions in Moldova and Georgia to Ukraine's east in a destabilizing southern corridor with Crimea in the middle.

Russia's Deputy Defence Minister Anatoly Antonov was quoted by the state's Itar-Tass news agency as saying Russia was complying with international agreements limiting the number of troops near its border with Ukraine.

Moscow's ambassador to the European Union, Vladimir Chizhov,

said Russia did not have "expansionist views". Asked to give a commitment that Russian troops would not move into Ukrainian territory outside Crimea, he told Britain's BBC. "There is no intention of the Russian Federation to do anything like that."

U.S. Senator John McCain, a Republican foreign policy specialist, told the same BBC show that Putin's actions in Ukraine were akin to those of Adolf Hitler in 1930s Germany.

"I think he (Putin) is calculating how much he can get away with, just as Adolf Hitler calculated how much he could get away with in the 1930s," McCain said.

"I'm very worried the unlawful attempt to alter recognized borders in our European neighbourhood, 25 years after the end of the Cold War, will open Pandora's Box," he said.

Belarussian President Alexander Lukashenko, a close ally of Russia, accepted on Sunday that Crimea was now "de facto" a part of Russia, but said the annexation set a "bad precedent".

Speaking to reporters in Minsk, Lukashenko said Ukraine, which shares a long land border with Belarus, should remain "a single, indivisible, integral, non-bloc state".

Western sanctions lost some of their sting on Sunday when Russia's SMP bank, whose main shareholders were targeted by U.S. sanctions, said Visa Inc and MasterCard Inc had resumed payment services for its clients.

The bank said it was glad the two biggest international payments systems had listened to its arguments to reverse Friday's suspension of services as it was wrong to target the bank, which was not itself subject to any sanctions.

A spokesperson for Mastercard confirmed it was again serving clients of the bank but did not say why it reversed its decision. Visa said it had been informed by the U.S. government to lift sanctions against SMP bank and two other Russian banks because they did not meet the criteria for sanctions.

Putin and Russian media had mocked the sanctions, which did not stop the Russian military completing its takeover of Ukraine's military bases in Crimea. Russia's defence ministry said on Sunday that its flag was now flying over 189 Ukrainian military installations on the peninsula.

A referendum held a week ago after Russian troops had seized control of Crimea overwhelmingly backed union with Russia but was denounced by Washington and the European Union as a sham.

The EU emphasized its support for the new pro-Western government in Kiev, signing a political agreement with interim Prime Minister Arseniy Yatseniuk last week.

It also promised financial aid for the government - which Moscow says came to power by a coup to overthrow Yanukovich after he rejected an EU trade deal in favor of closer ties with Russia - as soon as Kiev reaches a deal with the International Monetary Fund. The IMF will report on Tuesday.

The manner in which the President of Ukraine was toppled illegally, ostentatiously with Western assistance, it is quite rediculous to feel that Russia would be a partner and not an adversary!

This is another example of how the West without thinking through the issues, preempt the activities and then regret.

There could have been better ways to bring Ukraine into the West's sphere of influence, rather than make a mockery of democracy, which the West so glibly talk about when intervening into countries that are not of their choice.

Sandwiched between Romania and Ukraine, Moldova emerged as an independent republic following the collapse of the USSR in 1991.

The bulk of it, between the rivers Dniester and Prut, is made up of an area formerly known as Bessarabia. This territory was annexed by the USSR in 1940 following the carve-up of Romania in the Ribbentrop-Molotov pact between Hitler's Germany and Stalin's USSR.

Two-thirds of Moldovans are of Romanian descent, the languages are virtually identical and the two countries share a common cultural heritage.

The industrialised territory to the east of the Dniester, generally known as Trans-Dniester or the Dniester region, was formally an autonomous area within Ukraine before 1940 when the Soviet Union combined it with Bessarabia to form the Moldavian Soviet Socialist Republic.

This area is mainly inhabited by Russian and Ukrainian speakers. As people there became increasingly alarmed at the prospect of closer ties with Romania in the tumultuous twilight years of the Soviet Union, Trans-Dniester unilaterally declared independence from Moldova in 1990.

There was fierce fighting there as it tried to assert this independence following the collapse of the USSR and the declaration of Moldovan sovereignty. Hundreds died. The violence ended with the introduction of Russian peacekeepers. Trans-Dniester's independence has never been recognised and the region has existed in a state of lawless and corrupt limbo ever since.

The region reasserted its demand for independence and also expressed support for a plan ultimately to join Russia in a September 2006 referendum which was unrecognised by Chisinau and the international community.

It still houses a stockpile of old Soviet military equipment and a contingent of troops of the Russian 14th army. Withdrawal began under international agreements in 2001 but was halted when the Trans-Dniester authorities blocked the dispatch of weapons. Subsequent agreements to resume did not reach fruition as relations between Moscow and Chisinau cooled.

The Moldovan parliament granted autonomous status to the Turkic-language speaking Gagauz region in the southwest of the republic in late 1994. It has powers over its own political, economic and cultural affairs.

Moldova is one of the poorest countries in Europe and has a large foreign debt and high unemployment. It is heavily dependent on Russia for energy supplies, and Russia has not hesitated to take advantage of this fact as a way of exerting economic pressure on Moldova.

The Russian gas giant Gazprom cut the gas supply Moldova in January 2006 and January 2009 - for several weeks on the latter occasion - and in November 2012 Moscow issued an ultimatum telling Chisinau to withdraw from energy agreements with the EU or face losing discounts on Russian gas supplies from Russia.

The fact the Moldovan economy has traditionally been heavily dependent on the export of wine to Russia has also allowed Moscow to apply economic pressure by occasionally banning the import of Moldovan wine.